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indented_forest.rb

require 'yaml'

# StringForest = nil | (String, StringForest)[]
# String -> StringForest
def parse_indented_forest(str)
  str = str.gsub(/^( *)([^": \n][^":\n]*)$/, '\1"\2":')
  handle_includes YAML.load(str)
end

def handle_includes(tree)
  case included = tree.delete("include".freeze)
  when Hash
    handle_includes deep_merge(included, tree)
  when Array
    handle_includes included.inject(tree) {|t, inc| deep_merge(inc, t) }
  when nil
    tree.map {|k, v| v ? [k, handle_includes(v)] : k }
  else
    raise ArgumentError
  end
end

def deep_merge(a, b)
  a or return b
  b or return a
  a.merge(b) {|k, v, w| deep_merge(v, w) }
end

if $0 == __FILE__
  require 'pp'
  pp parse_indented_forest DATA.read
end

__END__
foo: &foo
  bar: &bar
    baz
    quu
  qux
hoge
  include: [*foo, *bar]
  bar
    moga
    mogo
    quu
      moo
  fuga